1. Field of the Invention:
This invention relates to a motor actuator which is provided in, for example, an air conditioner for an automobile.
2. Description of the Prior Art:
There is known a motor actuator which is equipped in, for example, an air conditioner for an automobile for controlling the amount of air leaving it or the angle of its flow. Its general construction is shown in FIGS. 4 and 5. It includes a switching device 21 having a plurality of switches 21a connected in parallel to a plurality of serially connected resistors 20. A comparison circuit 22 has an input connected to the switching device 21. A driving circuit 23 receives a control signal S from the comparison circuit 22. A motor 24 is driven by the driving circuit 23 to rotate a rotatable plate. A position sensor 25 is provided for detecting the position of the rotatable plate.
If one of the switches 21a is closed, a comparison signal V, which is defined by the voltage set by at least one resistor 20 ahead of the closed switch 21a, is inputted to the comparison circuit 22. The comparison circuit 22 outputs the control signal S to the driving circuit 23 so that the motor 24 may be driven to rotate the rotatable plate. When the rotatable plate has been rotated to a predetermined position, its position is detected by the position sensor 25 and the corresponding detection signal Sd is fed to the comparison circuit 22. The comparison circuit 22 compares the detection signal Sd with the comparison signal V and discontinues to output the control signal S. As a result, the motor 24 is stopped and the rotatable plate is fixed in its predetermined position.
The motor actuator as hereinabove described has, however, a number of drawbacks. It is expensive, as it includes a large number of parts. An electrical noise is likely to enter the comparison circuit and cause it to work erroneously. Therefore, it is impossible to ensure that the motor always be controlled accurately.